In general, an integrated circuit refers to an electrical circuit contained on a single monolithic chip containing active and passive circuit elements. As should be well understood in this art, integrated circuits are fabricated by diffusing and depositing successive layers of various materials in a preselected pattern on a substrate. The materials can include semiconductive materials such as silicon, conductive materials such as metals, and low dielectric materials such as silicon dioxide. The semiconductive materials contained in integrated circuit chips are used to form almost all of the ordinary electronic circuit elements, such as resistors, capacitors, diodes, and transistors.
Integrated circuits are used in great quantities in electronic devices such as digital computers because of their small size, low power consumption and high reliability. The complexity of integrated circuits range from simple logic gates and memory units to large arrays capable of complete video, audio and print data processing. Presently, however, there is a demand for integrated circuit chips to accomplish more tasks in a smaller space while having even lower operating voltage requirements.
Currently, the semiconductor industry is focusing its efforts on reducing dimensions within each individual integrated circuit in order to increase speed and to reduce energy requirements. The demand for faster and more efficient circuits, however, has created various problems for circuit manufacturers. For instance, a unique problem has emerged in developing equipment capable of testing, evaluating and developing faster chips. Timing errors and pulse deviations may constitute a greater portion of a signal period at higher speeds. As such, a need exists not only for devices capable of detecting these errors but also devices capable of characterizing and identifying the errors.
In the past, electronic measurement devices have been used to test integrated circuits for irregularities by making frequency and period measurements of a signal output from the circuit. Certain devices, known as time interval analyzers, can perform interval measurements, i.e. measurements of the time period between two input signal events, and can totalize a specific group of events. A time interval analyzer generally includes a continuous time counter and a continuous event counter. Typically, the device includes a measurement circuit on each of a plurality of measurement channels. Each channel receives an input signal. By directing a signal across the channels to a given measurement circuit so that the circuit receives two input signals, the circuit is able to measure the time interval between two events in the signals. Such devices are capable of making millions of measurements per second.
Measurement devices based exclusively on counters, however, are unable to directly measure time intervals. In very general terms, a counter refers to an electronic device that counts events, for example pulses, on an input signal. The measurement device also typically includes a frequency standard or clock to measure the time period during which the counter is activated. Thus, the measurement device measures the number of input signal events that occur over a known time period and, therefore, measures the frequency of the events. In other words, clocks contained in counters generate a signal at a known frequency which is then used to measure the frequency of other signals.
By measuring certain characteristics of a signal emitted by an integrated circuit, time interval analyzers and counter-based measurement devices can be used to detect timing errors that may be present within the circuit. This information can then be used to assist in developing an integrated circuit or for detecting defects in mass-produced circuits.
Timing errors on integrated circuit signals are generally referred to as "jitter." Jitter, broadly defined as a deviation between a real pulse and an ideal pulse, can be a deviation in amplitude, phase, and/or pulse width. Jitter typically refers to small, high frequency waveform variations caused by mechanical vibrations, supply voltage fluctuations, control-system instability and the like.
Instruments such as time interval analyzers, counter-based measurement devices and oscilloscopes have been used to measure jitter. In particular, time interval analyzers can monitor frequency changes and frequency deviation over time. In this manner, they not only detect jitter, but can also characterize jitter so that its source can be determined. Generally, however, conventional devices, including time interval analyzers, are too slow to provide reliable measurements at the speed and frequency of high-speed integrated circuits.